1. Field of the Invention
The present invention relates to a plasma addressed liquid crystal display device having a flat panel structure in which liquid crystal chambers are mutually overlapped on plasma chambers. More specifically, the present invention concerns an optimum technique of line sequential drive timing for a plasma addressed liquid crystal display device.
2. Description of the Prior Art
Referring now to FIG. 3, a general structure of the conventional plasma addressed liquid crystal display device will be briefly explained. It should be noted that the plasma addressed liquid crystal display device is disclosed in, for instance, Japanese Laid-open Patent Application No. 1-217396, corresponding to U.S. Pat. No. 5,077,553. As shown in this drawing, this liquid crystal display device includes a flat panel structure constructed of a liquid crystal chamber 101, a plasma chamber 102, and a common intermediate sheet 103 interposed between the liquid crystal chamber and the plasma chamber. The plasma chamber 102 is fabricated by employing a lower-sided glass substrate 104, and a stripe-shaped groove 105 is formed on a surface of the glass substrate 104. This groove 105 extends along, for example, a row direction of a matrix. The respective grooves 105 are tightly sealed by the intermediate sheet 103 to constitute discharge channels 106 which are individually separated from each other. Excitable gas atoms are filled into the tightly sealed discharge channels 106. A convex portion 107 for separating the adjacent grooves 105 has a role of a separating wall for sectioning the respective discharge channels 106. A pair of plasma electrodes 108 and 109 being positioned in parallel to each other are provided on a curved bottom portion of each of the grooves 105. These plasma electrodes function as an anode "A" and a cathode "K", which may excite gas atoms contained in the discharge channel 106 to be brought into the metastable state, so that plasma is produced. This discharge channel 106 becomes a unit of row scanning operation. On the other hand, the liquid crystal chamber 101 is constructed by employing an upper-sided glass substrate 110. This glass substrate 110 is positioned opposite to the intermediate sheet 103 via a predetermined space into which a liquid crystal layer 111 is held. On the inner surface of the glass substrate, a signal electrode D made of a transparent conductive film is formed in a stripe shape. This signal electrode D is positioned perpendicular to the discharge channel 106, and constitutes a unit of a column signal. A matrix-shaped liquid crystal pixel is defined at an intersecting portion between the column signal unit and the row scanning unit.
In the plasma addressed liquid crystal display apparatus with the above-described structure, the display drive operation is carried out by switching/scanning the display channels 106 in the line sequential mode for performing plasma display, and by applying the data pulse to the signal electrode D located on the side of the liquid crystal chamber in synchronism with this scanning operation. As to this point, a small explanation will now be made with reference to FIG. 4. FIG. 4 schematically shows only two sets of liquid crystal pixels included in the plasma addressed liquid crystal display device indicated in FIG. 3. Each of the liquid crystal pixels 112 is arranged by a series connection between a plasma sampling switch S1 and a sampling capacitor constructed of a liquid crystal layer 111 sandwiched by the signal electrodes D1, D2 and the intermediate sheet 103. The plasma sampling switch S1 is equivalently represented with the function of the display channel. That is, when the display channel is activated, an internal portion thereof is connected to the anode potential over the substantially entire channel. On the other hand, when plasma discharge channel is complete, the discharge channel is at the floating potential. The data pulse is written via the sampling switch S1 into the sampling capacitor of the each liquid crystal pixel 112 to perform a so-called "sampling hold" operation. The turn-ON or turn-OFF of the respective liquid crystal pixels 112 can be controlled in the gradation manner by the voltage levels of the data pulse.
FIG. 5 is a waveform chart for representing a selective pulse applied to a cathode of a discharge channel, and a data pulse applied to a signal electrode. When a selective pulse having a predetermined negative voltage "Vs" via a cathode "K" of one discharge channel at certain selective timing, plasma discharge is produced within the discharge channel. The selective pulse is released after a predetermined time period has passed. At the time instant when the application of the selective pulse is released, the cathode potential is returned to a predetermined reference potential Vo and then the plasma discharge is accomplished. It should be noted that the anode "A" is always set to the reference potential Vo. Since the gas atoms excited by the plasma discharge is still under metastable state at the releasing time instant of the selective pulse, the lower surface of the intermediate sheet made of a thin glass plate becomes conductive with the anode and the cathode, and thus is at the reference potential Vo. As a result, the signal voltage "V.sub.sig " of the data pulse is sampled at this releasing time instant of the selective pulse, and then a predetermined image signal can be written into the liquid crystal layer in accordance with the capacitance dividing ratio of the liquid crystal layer to the intermediate sheet. It should also be noted that the signal voltage "V.sub.sig " of the data pulse is set based upon the above-described reference potential Vo. Soon, the gas atoms under metastable state are returned to the ground state, and the resistance within the discharge channel become high while a small stray capacitance portion remains, so that the image signal written into the liquid crystal layer is maintained until the subsequent selective timing.
As previously described, the signal voltage "V.sub.sig " of the data pulse is set on the basis of the predetermined reference voltage Vo (anode potential). When the plasma discharge is produced, the potential at the lower surface of the intermediate sheet becomes substantially equal to the reference potential Vo (anode potential), and this signal voltage "V.sub.sig " is sampled, whereby the correct image signal is written into the liquid crystal pixel. However, the potential at the lower surface of the intermediate sheet is not always stable, but may be varied in response to recovery from the metastable state of the gas atoms to the ground state. Conventionally, since the potential at the lower surface of the intermediate sheet is not correctly set to the ground potential during the sampling operation of the data pulse, unnecessary DC voltage components are applied to the liquid crystal layer. Accordingly, there is a problem that the image display burning happens to occur.